Carbon-dioxide compound and catalyst

ABSTRACT

Tetrahedral [MoO 4 ] 2−  readily binds CO 2  to produce a robust [MoO 3 (κ 2 -CO 3 )] 2−  that can affect the reduction of CO 2  to formate.

PRIORITY CLAIM

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/917,980, filed Dec. 18, 2013, which is incorporated by referencein its entirety.

FIELD OF THE INVENTION

The invention features compounds including carbon dioxide and catalystsfor chemical transformations including carbon dioxide.

BACKGROUND

Although CO₂ chemical fixation cannot compete in scale with globalemissions, carbon dioxide is an attractive, low-cost, nontoxic, abundantchemical feedstock. See, R. J. Andres, T. A. Boden, F. -M. Breon, P.Ciais, S. Davis, D. Erickson, J. S. Gregg, A. Jacobson, G. Marland, J.Miller, T. Oda, J. G. J. Olivier, M. R. Raupach, P. Rayner and K.Treanton, Biogeosciences, 2012, 9, 1845-1871, M. Mikkelsen, M. Jørgensenand F. C. Krebs, Energy Environ. Sci., 2010, 3, 43-81., E. A. Quadrelli,G. Centi, J. -L. Duplan and S. Perathoner, ChemSusChem, 2011, 4,1194-215, I. Omae, Coord. Chem. Rev., 2012, 256, 1384-1405, A. Boddien,F. Gartner, C. Federsel, I. Piras, H. Junge, R. Jackstell and M. Beller,Organic Chemistry—Breakthroughs and Perspectives, Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim, Germany, 1st edn, 2012, ch. 18, pp. 685-724, andM. Holscher, C. Gurtler, W. Keim, T. E. Muller, M. Peters and W.Leitner, Z. Naturforsch, B: Chem. Sci., 2012, 67b, 961-975, each ofwhich is incorporated by reference in its entirety. Metal oxides areamong the most explored compounds for CO₂ capture and fixation, but arechallenging to study as they are of part of heterogeneous systems. See,B. Feng, H. An and E. Tan, Energy Fuels, 2007, 21, 426-434, S. Choi, J.H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796-854, S. Wang, K.Murata, T. Hayakawa. S. Hamakawa and K. Suzuki, Appl. Catal., A, 2000,196, 1-8, B. M. Bhanage, S. -I. Fujita, Y. Ikushima and M. Arai, Appl.Catal., A, 2001, 219, 259-266, and M. Matsuoka and M. Anpo, J.Photochem. Photobio., C, 2003, 3, 225-252, each of which is incorporatedby reference in its entirety,

SUMMARY

In one aspect, an isolated compound can include a molybdate complex ofcarbon dioxide.

In certain embodiments, the molybdate complex can include a singlemolybdenum atom. The molybdate complex can include a single carbonategroup. The molybdate complex can include [MoO₃(κ²-CO₃)]²⁻. The molybdatecomplex can include two carbonate groups. The molybdate complex caninclude [MoO₂(κ²-CO₃)₂]²⁻.

In certain embodiments, the isolated compound can include anon-coordinating cation. The non-coordinating cation can includebis(triphenylphosphine)iminium, an ammonium or a phosphonium.

In another aspect, a method of making an isolated molybdate complex ofcarbon dioxide can include exposing a molybdate to carbon dioxide, andisolating the molybdate complex of carbon dioxide.

In certain embodiments, the molybdate can be exposed to greater than oneatmosphere of carbon dioxide. The molybdate complex can have a formulaof [MoO₃(κ²-CO₃)]²⁻. The molybdate complex can have a formula of[MoO₂(κ²-CO₃)₂]²⁻.

In another aspect, a method for carbon dioxide fixation can includeexposing carbon dioxide to a molybdate in the presence of a mildnucleophile to produce a carbon dioxide-transformed product.

In another aspect, a method of sequestering carbon dioxide can includeexposing carbon dioxide to a molybdate in the presence of a mildnucleophile to produce a carbon dioxide-transformed product.

In certain embodiments, the mild nucleophile can include a mild hydridesource. The mild hydride source can include a silane or borane. The mildnucleophile can be an electron-rich alkene or alkyne. The mildnucleophile can be a metal hydride or metal alkyl. The mild nucleophilecan be an amine.

Other aspects, embodiments, and features will be apparent from thefollowing description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ⁹⁵Mo NMR resonance spectrum and solid-state structure of[NEt₄]₂[MoO₃(κ²-CO₃)] (ellipsoids at the 50% probability level, cationsomitted for clarity). Representative interatomic distances [Angstroms]and angles [°]: C1-O1 1.2258(13), C1-O2 1.3357(14), C1-O3 1.3048(13),Mo1-O2 2.0674(9), Mo1-O3 2.2191(9), Mo1-O4 1.7351(8), Mo1-O5 1.7390(8),Mo1-O6 1.7436(8); C1-O2-Mo1 97.75(6), C1-O3-Mo1 91.74(7).

FIG. 2 is a ¹³C NMR spectrum showing the distribution of[MoO₃(κ²-CO₃)]²⁻ and [MoO₂(κ²-CO₃)₂]²⁻ at −19° C. under 1 atmosphere of¹³CO₂ and a solid-state structure of [PPN]₂[MoO₂(κ²-CO₃)₂]. (ellipsoidsat the 50% probability level, cations and solvent molecules omitted forclarity). Representative interatomic distances [Angstroms] and angles[°]:C1-O3 1.302(4), C1-O4 1.352(4), C1-O5 1.228(4), C2-O6 1.303(3),C2-O7 1.358(3), C2-O8 1.223(3), Mo1-O1 1.695(2), Mo1-O2 1.705(3), Mo1-O32.198(2), Mo1-O4 2.024(2), Mo1-O6 2.175(2), Mo1-O7 2.0145(19); O3-C1-O4110.4(2), O6-C2-O7 109.9(2).

FIG. 3 is a ¹H NMR spectrum of [PPN]₂[MoO₄] in CD₃CN at 25° C.

FIG. 4 is a ¹³C NMR spectrum of [PPN]₂[MoO₄] in CD₃CN at 25° C.

FIG. 5 is a ⁹⁵Mo NMR spectrum of [PPN]₂[MoO₄] in CD₃CN at 25° C.

FIG. 6 is a ³¹P NMR spectrum of [PPN]₂[MoO₄] in CD₃CN at 25° C.

FIG. 7 is a ATR-IR spectrum of [PPN]₂[MoO₄].

FIG. 8 is a ¹H NMR spectrum of [PPN]₂[MoO₃(k²-CO₃)] in CD₃CN at 25° C.

FIG. 9 is a ¹³C NMR spectrum of [PPN]₂[MoO₃(k²-CO₃)] in CD₃CN at 25° C.

FIG. 10 is a ⁹⁵Mo NMR spectrum of [PPN]₂[MoO₃(k²-CO₃)] in CD₃CN at 25°C.

FIG. 11 is a ³¹P NMR spectrum of [PPN]₂[MoO₃(k²-CO₃)] in CD₃CN at 25° C.

FIG. 12 is a ATR-IR spectrum of [PPN]₂[MoO₃(k²-CO₃)].

FIG. 13 is a ¹H NMR spectrum of [NEt₄]₂[MoO₃(k²-CO₃)] in CD₃CN at 25° C.

FIG. 14 is a ⁹⁵Mo NMR spectrum of [NEt₄]₂[MoO₃(k²-CO₃)] in CD₃CN at 25°C.

FIG. 15 is a ¹³C NMR spectrum of [NEt₄]₂[MoO₃(k²-CO₃)] in CD₃CN at 25°C.

FIG. 16 is an ATR-IR spectrum of [NEt₄]₂[MoO₃(k²-CO₃)] in CD₃CN at 25°C.

FIG. 17 is a ¹H NMR spectrum of the crude reaction mixture containing[PPN][OCHO] and [PPN][MoO₄SiEt₃] in CD₃CN at 25° C.

FIG. 18 is a ¹H NMR spectrum of [PPN][OCHO] in CD₃CN at 25° C.

FIG. 19 is a ¹³C NMR spectrum of [PPN][OCHO] in CD₃CN at 25° C.

FIG. 20 is a ³¹P NMR spectrum of [PPN][OCHO] in CD₃CN at 25° C.

FIG. 21 is an ATR-IR spectrum of [PPN][OCHO].

FIG. 22 is a ¹H NMR spectrum of [PPN][MoO₄SiEt₃] in CD₃CN at 25° C.

FIG. 23 is a ¹³C NMR spectrum of [PPN][MoO₄SiEt₃] in CD₃CN at 25° C.

FIG. 24 is a ⁹⁵Mo NMR spectrum of [PPN][MoO₄SiEt₃] in CD₃CN at 25° C.

FIG. 25 is a ³¹P NMR spectrum of [PPN][MoO₄SiEt₃] in CD₃CN at 25° C.

FIG. 26 is an ATR-IR spectrum of [PPN][MoO₄SiEt₃].

FIG. 27 is a ¹³C NMR spectrum of [PPN]₂[MoO₃(k²-¹³CO₃)] in CD₃CN at 56°C.

FIG. 28 is a series of ¹H NMR spectra of the [PPN]₂[MoO₃(k²-¹³CO₃)]solution with increasing water content.

FIG. 29 is a series of ¹³C NMR spectra of the [PPN]₂[MoO₃(k²-¹³CO₃)]solution with increasing water content.

FIG. 30 is ATR-IR spectrum of [PPN]₂[MoO₃(k²-CO₃)] before and aftertreatment with water showing a carbonyl ban decrease and a molybdateband increase.

FIG. 31 depicts an evolution of the ATR-IR spectrum of[NEt4]2[MoO3(k2-CO3)] over the course of 15 minutes in air.

FIG. 32 is a ⁹⁵Mo NMR spectrum of the crude reaction mixture at 25 C

FIG. 33 is a ¹³C NMR spectrum of the initial reaction mixture.

FIG. 34 is a ¹³C NMR spectrum of the initial reaction mixture after[PPN][HCO₃] was added.

FIG. 35 is a ¹³C NMR spectrum of [PPN]₂[MoO₄] under 1 atmosphere of¹³CO₂ and after removing the ¹³CO₂.

FIG. 36 is a series of ¹³C NMR spectra of [PPN]₂[MoO₄] under 1atmosphere and 3 atmospheres of ¹³CO₂ at low temperature.

FIG. 37 is a ¹³C NMR spectrum of [PPN]₂[MoO₇] under 1 atmosphere of¹³CO₂ at low temperature.

FIG. 38 is a table containing x-ray crystallographic data for[NEt₄]₂[MoO₃(k²-CO₃)] and [PPN]₂[MoO₂(k²-CO₃)₂].

FIG. 39 is a combined calculated potential energy diagram for the firstand second CO₂ binding events.

FIG. 40 is a solid-state structure of [NEt₄]₂[WO₃(κ²-CO₃)] (ellipsoidsat the 50% probability level, cations omitted for clarity).

FIG. 41 is a ATR-IR spectrum of solid [NEt₄]₂[WO₃(κ²-CO₃)].

DETAILED DESCRIPTION

Terminal metal oxo species can form stable carbon dioxide complexes.Specifically, an isolated compound can include a molybdate complex ofcarbon dioxide or a tungstate complex of carbon dioxide. The molybdatecomplex can include a single molybdenum atom, derived, for example, from[MoO₄]²⁻. When exposed to carbon dioxide under conditions where anexcess of carbon dioxide is present, for example, two to threeequivalents of carbon dioxide, or more a complex forms that can beisolated. The isolated complex can include a single carbonate group, forexample, bonded to the complex through interaction with a terminal oxogroup to form a (κ²-CO₃)²⁻ group bound to the molybdenum. The molybdenumremains in a d⁰ state, or 6+ oxidation state.

Tetrahedral [MoO₄]²⁻ readily binds CO₂ to produce a robust[MoO₃(κ²-CO₃)]²⁻ that can affect the reduction of CO₂ to formate in thepresence of Et₃SiH. Under excess CO₂, a second molecule of CO₂ binds toafford [MoO₂(κ²CO₃)₂]²⁻, the first structurally characterized transitionmetal dicarbonate complex derived from CO₂.

Under certain circumstances, for example, when exposed to excess freecarbon dioxide, a second carbon dioxide adduct can form, leading to two(κ²-CO₃)²⁻ groups bound to the molybdenum. Any excess free carbondioxide in solution will form the bis-carbonate. The evidence for thepresence of the bis-carbonate in solution at room temperature underlittle excess carbon dioxide is the broad ¹³C NMR signal for themonocarbonate, which is exchanging ¹³C with the free carbon dioxide viathe bis-carbonate. The broadening of the signal is not explained byexchange happening via the dissociation of the carbon dioxide from themolybdate, as this process is unobserved even under heating at 56° C.Under certain conditions, such as temperature of −40° C. and 1atmosphere of CO₂, the bis-carbon dioxide complex can be isolated.

The compound includes a non-coordinating cation. For example, thenon-coordinating cation can be bis(triphenylphosphine)iminium, ammoniumor phosphonium. The ammonium can be a monoalkyl, dialkyl, triakyl,tetraalkyl or aryl ammonium. The phosphonium can be monoalkyl, dialkyl,triakyl, tetraalkyl or aryl phosphonium. In each case, the alkyl,independently, can be a C1-C16 alkyl group, preferably, a C1-C8 alkylgroup, which can be optionally substituted. The aryl can be phenyl orsubstitued phenyl. For example, the cation can betriphenylmethylphosphonium. The non-coordinating cation can be a lightlycoordinating cation. Another example of a suitable cation can be analkali metal ion sequestered with a crown ether (for example, an18-crown-6 potassium). The isolated compound can be formed by exposing amolybdate to carbon dioxide; and isolating the molybdate complex ofcarbon dioxide. The molybdate can be a salt including [MoO₄]²⁻ dissolvedin a suitable solvent. The carbon dioxide can be exposed asstoichiometric amount or higher of carbon dioxide gas. Alternatively,the molybdate in the solid state can react with carbon dioxide to formthe complex.

The molybdate, [MoO₄]²⁻, can be used to sequester carbon dioxide andstore it as the isolated compound. Alternatively, the molybdate,[MoO₄]²⁻, in the presence of carbon dioxide or the isolated carbondioxide complex itself, can be used to activate the carbon dioxide forreduction with a mild nucleophile. For example, the mild nucleophile canbe a mild hydride source, such as, a silane, for example, trimethylsilane, triethyl silane, a borane, for example, pinacolborane, or othermild hydride source. This reaction can produce formate from carbondioxide. In another example, the mild nucleophile can be anelectron-rich alkene or alkyne, such as, for example, a silyl enolether, a metal hydride, a metal alkyl or an amine, such as an aromaticamine or diamine, for example, phenyl amine or ortho-phenylenediamine.This transformation and activity can be useful in deoxygenation ofcarbon dioxide. See, for example, Berkefield et al., J. Am. Chem. Soc.2010, 132, 10660-10661, which is incorporated by reference in itsentirety. The molybdate can be used to provide carbon dioxide to anyprocess in need of a carbon dioxide source. The sequestering of carbondioxide and subsequent reaction to form a carbon dioxide-transformedproduct can be direct or indirect. For example, the carbon dioxidesequestering can take place in the same vessel as a second chemicalprocess for direct transformation. In another example, the molybdate canbind carbon dioxide and that complex can later be used in a secondchemical process for indirect transformation. Advancing theunderstanding of CO₂ binding and reactivity is essential for developingnew uptake technologies. Metal oxides are among the most exploredcompounds for CO₂ capture and fixation, but are challenging to study asthey are oftentimes part of heterogeneous systems. Having molecularmodels to study the reactivity of metal oxides with CO₂ in solution ishighly desirable. Disclosed herein is a simple metal oxo platforminspired by CO₂ binding using a titanium oxo anion. See, J. S. Silviaand C. C. Cummins, Chem. Sci., 2011, 2, 1474-1479, which is incorporatedby reference in its entirety. The molybdate dianion was a top candidatedue to its simple structure, high nucleophilicity, ease of access andlow cost. See, J. R. Briggs, A. M. Harrison and J. H. Robson,Polyhedron, 1986, 5, 281-287, J. E. Hamlin and M. J. Lawrenson, Processfor the production of either an alkylene carbonate, a glycol ether esteror a glycol ether, GB2187454A, 1987, B. Wikjord and L. D. Byers, J. Am.Chem. Soc., 1992, 114, 5553-5554, B. R. Wikjord and L. D. Byers, J. Org.Chem., 1992, 57, 6814-6817, C. Polydore, D. Roundhill and H. -Q. Liu, J.Mol. Catal. A: Chem., 2002, 186, 65-68, and D. V. Partyka and R. H.Holm, Inorg. Chem., 2004, 43, 8609-8616, each of which is incorporatedby reference in its entirety. Selective CO₂ uptake has been reported fora metal-organic material containing [MoO₄]²⁻ pillars, a zirconiumκ²-carbonate was prepared from a zirconium oxo complex and CO₂, and thetungstate dianion was shown to be an efficient catalyst for carbondioxide fixation with challenging organic substrates such as aromaticdiamines or 2-aminobenzonitriles. See, M. H. Mohamed, S. K. Elsaidi, L.Wojtas, T. Pham, K. a. Forrest, B. Tudor, B. Space and M. J. Zaworotko,J. Am. Chem. Soc., 2012, 134, 19556-19559, J. P. Krogman, M. W.Bezpalko, B. M. Foxman and C. M. Thomas, Inorg. Chem., 2013, 52,3022-3031, T. Kimura, K. Kamata and N. Mizuno, Angew. Chem. Int. Ed.,2012, 51, 6700-6703, and T. Kimura, H. Sunaba, K. Kamata and N. Mizuno,Inorg. Chem., 2012, 51, 13001-13008, each of which is incorporated byreference in its entirety.

Here, a procedure for preparing [PPN]₂[MoO₄] (PPN⁺=(Ph₃P)₂N⁺) in onestep from Ag₂MoO₄ and PPNCl is disclosed that overcomes the limitationof the commercially available sodium molybdate that is practicallyinsoluble in most organic solvents and does not react with CO₂ underaqueous conditions. A new species quickly formed upon addition of CO₂ toa solution of [PPN]₂[MoO₄] at room temperature. ⁹⁵Mo NMR spectrum of theproduct mixture reveals a new resonance at +46.7 ppm, no more startingmaterial (+13.2 ppm), but also a small amount of [Mo₂O₇]²⁻ by-product at−3.8 ppm. A new characteristic carbonyl stretch at 1638 cm⁻¹ could alsobe observed by IR. See G. Busca and V. Lorenzelli, Mater. Chem., 1982,7, 89-126.

A preliminary X-ray crystal structure of the CO₂-addition productrevealed a κ²-bound carbonate moiety and enabled formulation of themajor product obtained as [PPN]₂[MoO₃(κ²-CO₃)]. Unfortunately, crystalsobtained with the PPN counterion were low quality. Since [NEt₄]₂[MoO₄]had been previously reported in the literature and used to prepare a fewcrystallographically characterized molybdenum complexes, this startingmaterial was used to obtain the [MoO₃(κ²-CO₃)]²⁻ dianion as itstetraethylammonium salt.

X-ray diffraction quality crystals were grown by slow vapor diffusion ofEt₂O into a CH₃CN solution of [NEt₄]₂[MoO₃(κ²-CO₃)], and the resultingstructure is shown in FIG. 1. The C—O distances are elongated from 1.162Angstroms in free CO₂ to 1.2258(13) Angstroms, 1.3048(13) Angstroms, and1.3357(14) Angstroms in the carbonate unit. The average Mo—O distance is1.739 Angstroms for the three molybdenum oxo bonds, shorter than theaverage Mo—O distance of 1.776 Angstroms in tetrahedral [MoO₄]²⁻. Thecarbonate ligand has more elongated Mo—O bonds of 2.0674(9) and2.2191(9) Angstroms. The slight asymmetry of the carbonate induced bythe trans influence of one of the molybdenum oxo ligands is reflected inthe C—O bond lengths that differ by approximately 0.031 Angstroms, butalso in the different Mo—O—C angles of 97.75(6) and 91.74(7)°. This κ²binding mode is not surprising given the lack of steric bulk around themolybdenum center, in contrast to [O₂COTiX₃]⁻ (X=N[′Bu](3,5-Me₂C₆H₃))forwhich a combination of ancillary ligand steric bulk and externalcarbonate complexation by an alkali-metal counter-ion promotesκ¹-binding of CO₃ ²⁻ to the titanium center.

Solid [PPN]₂[MoO₃(κ²-CO₃)] is moderately stable in air, and does notlose CO₂ even after being heated at 70° C. under vacuum for 1 h. Insolution, [PPN]₂[MoO₃(κ2-¹³CO₃)] was heated to 56° C. without anyobservable loss of ¹³CO₂ as confirmed by ¹³C NMR spectroscopy. However,this compound is moisture sensitive, as it converts to [PPN]₂[MoO₄] wheneven a few equivalents of water is added to a solution of[PPN]₂[MoO₃(κ²-CO₃)]. On the other hand, solid [NEt₄]₂[MoO₃(κ²-CO₃)] ishygroscopic and undergoes decomposition in ca. 15 minutes by absorbingmoisture from the ambient atmosphere.

[PPN]₂[MoO₃(κ²-¹³CO₃)] prepared and isolated using ¹³CO₂ displays asharp carbonate resonance at 165.7 ppm by ¹³C NMR spectroscopy, a regioncharacteristic for carbonates. See, J. S. Silvia and C. C. Cummins,Chem. Sci., 2011, 2, 1474-1479, J. P. Krogman, M. W. Bezpalko, B. M.Foxman and C. M. Thomas, Inorg. Chem., 2013, 52, 3022-3031, and D. J.Darensbourg, K. M. Sanchez and A. L. Rheingoldib, J. Am. Chem. Soc.,1987, 109, 290-292, each of which is incorporated by reference in itsentirety. In its IR spectrum, an isotope-shifted carbonyl stretch ispresent at 1599 cm⁻¹, very close to the theoretical 1602 cm⁻¹ predictedbased on the stretch of [PPN]₂[MoO₃(κ²-CO₃)] at 1638 cm⁻¹. A small peakat 158.9 ppm could also be observed by ¹³C NMR, correlated with thetrace dimolybdate by-product detected by ⁹⁵Mo NMR spectroscopy. Adding[PPN][HCO₃] to a mixture of [PPN]₂[MoO₃(κ²-¹³CO₃)] and this unknownspecies yielded an increase in the unknown peak and no additionalresonances, allowing us to identify [HCO₃]⁻ as the by-product. See, M.L. Meckfessel Jones, PhD thesis, Texas A&M University, 1994, which isincorporated by reference in its entirety. It is likely that during thesynthesis of [PPN]₂[MoO₃(κ²-CO₃)]²⁻, a small amount of unreacted[MoO₄]²⁻ can attack a [MoO₃(κ²-CO₃)]²⁻ molecule to yield [Mo₂O₇]²⁻ andfree [CO₃]²⁻, the latter then picking up a proton from adventitiouswater to become [HCO₃]⁻.

Under 1 atmosphere of ¹³CO₂, the room temperature ¹³C NMR spectrum of a[PPN]₂[MoO₄] solution reveals 2 broad signals: one for the free ¹³CO₂ at125.8 ppm, and one for the molybdenum carbonate at 165.3 ppm, indicatingthat a fast chemical exchange is occurring. A new major resonanceappeared at 162.8 ppm when acquiring the spectrum at −19° C., butdisappeared after degassing the sample. The ratio of the unknown speciesat 162.8 ppm to [MoO₃(κ²-¹³CO₃)]²⁻ increases at higher pressure of ¹³CO₂(3 atmospheres), and at lower temperatures (−31° C.). These data areindicative of additional reversible binding of the ¹³CO₂ to the[MoO₃(κ²-¹³CO₃)]²⁻.

The existence of a [PPN]₂[MoO₂(κ²-CO₃)₂] species was confirmed by X-raycrystallography (FIG. 2), as diffraction quality crystals were grown byslowly cooling a CH₃CN solution of [PPN]₂[MoO₄] under an atmosphere ofCO₂. In the solid state, both carbonates are bound κ², with Mo—Odistances of 2.198(2) and 2.024(2), 2.175(2) and 2.0145(19) Å for thetwo carbonate ligands, respectively. The molybdenum oxo distances are1.695(2) and 1.705(3) Å, shorter than in [MoO₃(κ²-CO₃)]²⁻ as the Mo—O πcharacter is shared over fewer centers. The carbonates exhibit the sametype of asymmetry as in [MoO₃(κ²-CO₃)]²⁻ due to the trans influence ofthe oxo ligands. Several examples of κ²-bound molybdenum carbonates arereported in the Cambridge Structural Database (see, J. Chatt, M. Kubota,G. J. Leigh, F. C. March, R. Mason and D. J. Yarrow, J. Chem. Soc.,Chem. Commun., 1974, 1033-1034, E. Carmona, F. Gonzalez, M. L. Poveda,J. M. Marin, J. L. Atwood and R. D. Rogers, J. Am. Chem. Soc., 1983,105, 3365-3366, D. M. Curtis and K. R. Han, Inorg. Chem., 1985, 24,378-382, K. A. Belmore, R. A. Vanderpool, J. -C. Tsai, M. A. Khan and K.M. Nicholas, J. Am. Chem. Soc., 1988, 110, 2004-2005, and L. Contreras,M. Paneque, M. Sellin, E. Carmona, P. J. Perez, E. Gutierrez-Puebla, A.Monge and C. Ruiz, New J. Chem., 2005, 29, 109-115, each of which isincorporated by reference in its entirety), this is the first example ofa molybdenum complex with two κ²-carbonates. [PPN]₂[MoO₂(κ²-CO₃)₂] isalso the first transition metal dicarbonate complex obtained directlyfrom CO₂. See, S. V. Krivovichev and P. C. Burns, Radiochemistry, 2004,46, 12-15. 33 Y. Do, E. D. Simhon and R. H. Holm, Inorg. Chem., 1985,24, 1831-1838, which is incorporated by reference in its entirety.

To see whether molybdenum carbonate can act as a source of activatedCO₂, molybdenum carbonate was subjected to a mild hydride source such astriethylsilane, which exhibits no background reactivity with CO₂. A testreaction revealed a new resonance at 8.73 ppm by ¹H NMR spectroscopy, aregion characteristic for formyl protons, and also a new species by ⁹⁵MoNMR spectroscopy at −23.7 ppm, a shift essentially identical to thatreported for the [MoO₃(OSiMe₃)]³¹ anion. See, Y. Do, E. D. Simhon and R.H. Holm, Inorg. Chem., 1985, 24, 1831-1838, which is incorporated byreference in its entirety. The conversion to the formate improvesdramatically if the reaction is run under an atmosphere of CO₂, whichraises the question whether the active species that enables CO₂reduction is [MoO₃(κ²-CO₃)]²⁻ or [MoO₂(κ²-CO₃)₂]²⁻. After a briefoptimization, clean conversion to [PPN][OCHO] and [PPN][MoO₄SiEt₃] asthe sole products can be obtained after 22 h at 85° C., as evidenced bythe ¹H NMR spectrum of the crude reaction. From this reaction mixture,we were able to isolate [PPN][OCHO] in 69% yield, along with[PPN][MoO₄SiEt₃] in 50% yield based on their different solubilities inTHF.

In summary, two molybdenum oxo carbonate species obtained from theuptake of CO₂ by the molybdate dianion were structurally characterizedand their reactivity was explored in the context of CO₂ reduction toformate. ¹³C-labeling experiments suggest that the first binding eventto form [MoO₃(κ²-CO₃)]²⁻ is irreversible, while the second CO₂ moleculebinds reversibly.

General Method

All manipulations were performed using standard Schlenk techniques or ina nitrogen atmosphere glovebox, unless otherwise stated. All reagentswere purchased from Aldrich or Alfa Aesar. Ag₂MoO₄ was prepared fromNa₂MoO₄•2 H₂O following a literature procedure. See, C. Rosner and G.Lagaly, J. Solid State Chem., 1984, 53, 92-100, which is incorporated byreference in its entirety. [PPN][HCO₃] was also prepared using areported synthesis. See, M. L. Meckfessel Jones, Ph.D. thesis, Texas A&MUniversity, 1994, which is incorporated by reference in its entirety.[NEt₄]₂[MoO₄] was synthesized from commercially available [NEt₄]OH andH₂MoO₄ following a literature procedure for a similar compound.[NnBu₄]₂[WO₄]. See, T. M. Che, V. W. Day, L. C. Francesconi, M. F.Fredrich, W. G. Klemperer and W. Shum, Inorg. Chem., 1985, 24,4055-4062, which is incorporated by reference in its entirety. Solvents(EMD Chemicals) were either used as received or purified on a GlassContour Solvent Purification System built by SG Water USA, LLC. IRspectra were recorded on a Bruker Tensor 37 Fourier transform IR (FTIR)spectrometer. Elemental analyses were performed by Robertson MicrolitLaboratories, Inc. Low-temperature X-ray diffraction data were collectedon a Broker X8 Kappa DUO four-circle diffractometer coupled to a BrukerSmart APEX2 charged coupled device (CCD) detector. The structures weresolved by direct methods using SHELXS-97 or intrinsic phasing usingSHELXT and refined against F² all data by full-matrix least squares withSHELXL-2012 using established methods. All non-hydrogen atoms wererefined anisotropically. NMR solvents were obtained from CambridgeIsotope Laboratories, and NMR spectra were obtained on a Bruker 400 MHzspectrometer. For low temperature experiments, all reported temperatureshave been calibrated using a methanol standard. ¹H NMR and ¹³C{¹H} NMRare referenced to residual protio-solvent signals, ⁹⁵Mo NMR spectra arereferenced to a 2 M Na₂MoO₄ in D₂O external standard, and ³¹P NMRspectra are referenced to a 85% H₃PO₄ external standard.

Synthesis of [PPN]₂[MoO₄]

Outside the glovebox, Ag₂MoO₄ (9.93 g, 26.4 mmol, 1.1 equiv) and PPNCl(27.56 g , 48 mmol, 2 equiv) were weighed and transferred to a 2 L roundbottom flask. 900 mL of 2:1 CH₃CN:H₂O (pH 8) solvent mixture was addedto the solids, the flask was sealed with a septum, and shielded fromlight with aluminum foil. The slurry was stirred vigorously at roomtemperature for 24 h. After 24 h, the slurry was filtered through a padof Celite, then the filtrate was concentrated to 200 mL using a rotaryevaporator. Most of the remaining water was removed under vacuum on theSchlenk line. The thick slurry (ca. 80 mL water left) was filtered on amedium porosity frit and washed with 2×40 mL of cold water (pH 8). Thewhite solid was briefly dried on the frit, then transferred to a flaskand dried under vacuum overnight, after which it was slurried for 30minutes in 130 mL of dry Et₂O, filtered, and dried on the frit. Theresulting powder was transferred to a 500 mL round bottom flask anddried under vacuum overnight. The solid was split into smaller batchesdried under vacuum at room temperature over P₂O₅. 23.31 g of materialwere obtained (79% yield).

Characterization of [PPN]₂[MoO₄] (FIGS. 3-7)

¹H NMR (CD₃CN, 25° C., 400.1 MHz) δ: 7.67 (1 H, m), 7.57 (2 H, m), 7.48(2 H, m) ppm. ¹³C{¹H} NMR (CD₃CN, 25° C., 100.6 MHz) δ: 134.6 (s), 133.2(m), 130.4 (m), 128.2 (d, ¹JPC−108 Hz) ppm.

⁹⁵Mo NMR (CD₃CN, 25° C., 26.1 MHz) δ: 13.2 ppm.

³¹P{¹H} NMR (CD₃CN, 25° C., 162.0 MHz) δ: 21.96 ppm. ATR-IR: 3045, 1639,1586, 1481, 1436, 1282, 1241, 1184, 1110, 1026, 996, 811, 788, 753, 721,690, 615 cm⁻¹.

Elemental analysis [%] found (calculated for C₇₂H₆₀MoN₂O₄P₄): C, 69.72(69.90); H, 4.71 (4.89); N, 2.09 (2.26).

Synthesis of [PPN]₂[MoO₃(κ²-CO₃)]

[PPN]₂[MoO4] (1.24 g, 1 mmol, 1 equiv) was dissolved in 25 mL of CH₃CNand transferred to a round bottom flask that was then capped with aseptum. The flask was taken outside the glovebox where 60 mL (2.5 mmol,2.5 equiv) of CO₂ were bubbled through the solution. After 5 min, thesolvent was removed under vacuum, after which the flask was brought backinside the glovebox and the white solid triturated with 3×5 mL of Et2O.1.15 g of white solid were obtained (93% yield). The product obtainedthrough this method is a 18:1 mixture (assessed by ⁹⁵Mo NMR) of[PPN]₂[MoO₃(κ²CO₃)] and [PPN]₂[Mo₂O₇]. We were unable to separate thetwo compounds because of their very similar solubility properties.

Characterization of [PPN]₂[MoO₃(κ²-CO₃)] (FIGS. 8-12)

¹H NMR (CD₃CN, 25° C., 400.1 MHz) δ: 7.66 (1 H, m), 7.57 (2 H, m), 7.48(2 H, m) ppm.

¹³C{¹} NMR (CD₃CN, 25° C., 100.6 MHz) δ: 134.6 (s), 133.2 (m), 130.4(m), 128.2 (d, ¹JPC=108 Hz) ppm.

⁹⁵Mo NMR (CD₃CN, 25° C., 26.1 MHz) δ: 46.7 ([MoO₃(κ²-CO₃)]²⁻), −3.8([Mo₂O₇]²⁻) ppm. ³¹P{¹H} NMR (CD₃CN, 25° C., 162.0 MHz) δ: 21.93 ppm.

ATR-IR: 3050, 1676, 1638 (s), 1586, 1481, 1436, 1247, 1180, 1163, 1109,1022, 997, 903, 836, 800, 760, 750, 721, 689, 615 cm⁻¹

Elemental analysis [%] found (calculated for C₇₃H₆₀MoN₂O₆P₄): C, 68.71(68.44); H, 4.70 (4.72); N, 1.98 (2.19).

Synthesis of [NEt₄]₂[MoO₃(κ₂-CO₃)]

[NEt₄]₂[MoO₄ ] (840 mg, 2 mmol, 1 equiv) was dissolved in 40 mL of CH₃CNand transferred to a Schlenk flask that was then capped with a septum.The flask was taken outside the glovebox where 120 mL (5 mmol, 2.5equiv) of CO₂ were bubbled through the solution. After 10 min, thesolvent was removed under vacuum, after which the flask was brought backinside the glovebox. 800 mg of white solid were obtained (86% yield).

Characterization of [NEt₄]₂[MoO₃(κ₂-CO₃)] (FIGS. 13-16)

¹H NMR (CD₃CN, 25° C., 400.1 MHz) δ: 3.26 (2 H, q, ³J_(HH)=7.3 Hz), 1.22(3 H, tt, ³J_(HH)=7.3 Hz, ²J_(HH)=1.8 Hz) ppm.

¹³C{¹H} NMR (CD₃CN, 25° C., 100.6 MHz) δ: 165.6, 53.1, 7.9 ppm.

⁹⁵Mo NMR (CD₃CN, 25° C., 26.1 MHz) δ: 46.4 ppm.

ATR-IR: 2984, 1628 (s), 1453, 1394, 1265, 1185, 1019, 1005, 901, 839,807, 757, 676, 626 cm⁻¹

Elemental analysis [%] found (calculated for C₁₇H₄₀MoN₂O₆): C, 43.71(43.96); H, 8.93 (8.68); N, 6.27 (6.03).

Reaction of [PPN]₂[MoO₃(κ²-CO₃)] with Et₃SiH

[PPN]₂[MoO₃(κ²-CO₃)] (513 mg, 0.4 mmol, 1 equiv) was dissolved in 6 mLof CH₃CN and transferred to a Schlenk tube, followed by triethylsilane(140 mg, 1.2 mmol, 3 equiv) dissolved in 4 mL of CH3CN. The Schlenk tubewas sealed and taken outside the glovebox, where it was connected to theSchlenk line and degassed using 4 freeze-pump-thaw cycles. The tube wasthen refilled with CO₂ (1 atmosphere) from the manifold, closed, andheated in 85° C. overnight (22 h). The solvent was removed under vacuumand the flask was brought back into the glovebox. The crude residue wastriturated with Et₂O (5×8 mL) to remove traces of CH₃CN and obtain anoff-white powder. This crude solid was placed on a frit and washed with3×3 mL THF. The THF insoluble white solid collected was analyzed by NMRand IR and confirmed to be [PPN][OCHO] (162 mg, 69% yield). The THFfiltrate was concentrated and triturated with ether (3×3 mL). 284 mg ofproduct were collected, and confirmed to be [PPN][MoO₄SiEt₃] with asmall amount of [PPN][OCHO] contamination. The product was crystallizedby slow cooling of a 3:1 THF:Et2O solution, filtered, and washed with3×1 mL Et2O to give 161 mg [PPN][MoO₄SiEt₃] (50% yield).

FIG. 16 shows ¹H NMR of the crude reaction mixture containing[PPN][OCHO] and [PPN][MoO₄SiEt₃] in CD₃CN at 25° C.

Characterization of [PPN][OCHO] (FIGS. 17-21)

¹H NMR (CD₃CN, 25° C., 400.1 MHz) δ: 8.73 (s, 1 H, ¹J_(CH)=86.1 Hz),7.66(6 H, m), 7.57 (12 H, m), 7.48 (12 H, m) ppm.

¹³C{¹H} NMR (CD₃CN, 25° C., 100.6 MHz) δ: 166.4 (s), 134.6 (s), 133.2(m), 130.4 (m), 128.2 (¹JPC=108 Hz) ppm.

³¹P{¹H} NMR (CD₃CN, 25° C., 162.0 MHz) δ: 22.01 ppm.

ATR-IR: 3056, 2594, 1600 (vs), 1584, 1482, 1435, 1365, 1333, 1287, 1264,1186, 1162, 1111, 1026, 997, 797, 765, 745, 722, 692, 664, 616 cm⁻¹

Elemental analysis [%] found (calculated for C₃₇H₃₁NO₂P₂): C, 73.43(76.15); H, 4.96 (5.35); N, 2.28 (2.40).

Characterization of [PPN][MoO₄SiEt₃] (FIGS. 22-26)

¹H NMR (CD₃CN, 25° C., 400.1 MHz) δ: 7.66 (6 H, m), 7.58 (12 H, m), 7.48(12 H, m), 0.95 (9 H,t, ³J_(HH)=7.9 Hz), 0.55 (6 H, q, ³J_(HH)=7.9 Hz)ppm.

¹³C{¹H} NMR (CD₃CN, 25° C., 100.6 MHz) δ: 134.6 (s), 133.2 (m), 130.4(m), 128.2 (d, ¹JPC=108 Hz), 7.23 (s), 7.04 (s) ppm.

⁹⁵Mo NMR (CD₃CN, 25° C., 26.1 MHz) δ: −23.65 ppm.

³¹P{¹H} NMR (CD₃CN, 25° C., 162.0 MHz) δ: 22.00 ppm.

ATR-IR: 3056, 2955, 2874, 1588, 1483, 1460, 1436, 1284, 1260, 1184,1160, 1113, 998, 950, 878 (vs), 800, 745, 721, 691, 616 cm⁻¹

Elemental analysis [%] found (calculated for C₄₂H₄₅MoNO₄P₂Si): C, 61.94(61.99); H, 5.30 (5.57); N, 1.64 (1.72).

Stability Studies on [PPN]₂[MoO₃(κ²-CO₃)]

Thermal Stability of [PPN]₂[MoO₃(κ²-CO₃)]

In the solid state: A small amount (ca. 30 mg) of [PPN]₂[MoO₃(κ²-CO₃)]were placed in a Schlenk flask and brought outside the glovebox. Theflask was heated under dynamic vacuum in an oil bath at 70° C. for 1hour. The resulting solid was analyzed by ATR-IR, showing an identicaltrace to the initial material with no decomposition or decrease in thecarbonyl band.

In solution: 30 mg of [PPN]₂[MoO₃(κ²-¹³CO₃)] were dissolved in ca. 0.6mL of CD₃CN and transferred to an NMR tube glass blown onto a 14/20female adapter. A vacuum adapter was added, and the sealed system wasbrought outside the box. The solution was frozen in liquid nitrogen, thesystem was evacuated, then the tube was flame sealed. ¹³C NMR spectra(e.g., FIG. 27) were taken at room temperature, 38° C. and 56° C. Nofree ¹³CO₂ was observed in solution. In contrast to the broad signalobserved in the presence of excess ¹³CO₂, the signal of[MoO₃(κ²-¹³CO₃)]²⁻ remained sharp, indicating that no chemical exchangewas occurring.

Addition of Water to [PPN]₂[MoO₃(κ²-¹³CO₃)] Monitored by ¹³C NMR

[PPN]₂[MoO₃(κ²-¹³CO₃)] (26 mg, 0.02 mmol, 1 equiv) was dissolved in ca.0.6 mL of CD₃CN, transferred to an NMR tube, and capped with a septum. Astock solution of H₂O in CH₃CN was prepared by adding 36 μL H₂O to ca.0.97 mL CH₃CN. 1H NMR and ¹³C NMR spectra of the initial sample weretaken. Stoichiometric amounts of water were added via microsyringe fromthe stock solution in 100 μL increments. Each 100 μL of the stocksolution contain 0.02 mmol (1 equiv) H₂O. ¹H NMR and ¹³C NMR spectrawere collected after the addition of 1, 2, 4.5 and 9 equiv of water. The¹H NMR spectra were used to confirm the stoichiometry (FIG. 28). The ¹³CNMR spectra were diagnostic for the stability of the[PPN]₂[MoO₃(κ²-¹³CO₃)] (FIG. 29). Although in a decreased amount,[PPN]₂[MoO₃(κ²-¹³CO₃)] is present in solution even after the addition of9 equivalents of water.

Addition of Water to [PPN]₂[MoO₃(κ²-CO₃)] monitored by ATR-IR

A small amount of [PPN]₂[MoO₃(κ²-CO₃)] (20 mg, 0.0156 mmol) wasdissolved in 1 mL CH₃CN in a vial that was then capped with a septum.The vial was taken outside the glovebox where 10 μL (10 mg, 0.555 mmol)of water were injected using a microsyringe. The solution was stirredfor ca. 10 min, the volatiles were removed, and the residue analyzed bATR-IR. FIG. 30 shows ATR-IR of [PPN]₂[MoO₃(κ²-CO₃)] before and aftertreatment with water showing a carbonyl ban decrease and a molybdateband increase.

Hygroscopy of [NEt₄]₂[MoO₃(κ²-CO₃)] Monitored by ATR-IR

A small amount (ca. 5 mg) of [NEt₄]₂[MoO₃(κ²-CO₃)] was placed on the ATRplate and the IR spectrum of this sample was recorded. Additionalspectra of the same sample were recorded every 2-3 minutes for a totalof 15 minutes, time during which the sample remained in air on the ATRplate and was absorbing atmospheric moisture. The overlay below showsthe correlation between the disappearance of the carbonate band and thegrowth of the broad water OH band. The end product displays bands in themetal oxo region of the spectrum corresponding to molybdate anddimolybdate. See FIG. 31.

Formation of [PPN]₂[MoO₃(κ²-CO₃)] in Wet Aerobic Conditions

[PPN]₂[MoO₄] (100 mg, 0.08 mmol, 1 equiv) was dissolved in non-degassedACS grade acetonitrile (2 mL) under air. Carbon dioxide (40 mL, 1.64mmol, 20 equiv) was bubbled through the stirring molybdate solution. Thereaction was allowed to stir open to air for 10 minutes, after which analiquot was taken for NMR analysis. The ⁹⁵Mo NMR spectrum of the crudereaction mixture indicates formation of [PPN]₂[MoO₃(κ²-CO₃)] and[PPN]₂[Mo₂O₇] in a 1.6:1 ratio. See FIG. 32.

¹³C-Labeling Experiments

Identifying HCO3⁻ in the ¹³C NMR of [PPN]₂[MoO₃(κ²-¹³CO₃)] (FIGS. 33-34)

[PPN]₂[MoO₄] (30.4 mg, 0.025 mmol, 1 equiv) was dissolved in ca. 0.6 mLof CD₃CN, transferred to an NMR tube, and capped with a septum. The tubewas taken outside the glovebox and the solution submitted to 3freeze-pump-thaw cycles. ¹³CO₂ (0.6 mL, 0.025 mmol, 1 equiv) was addedby syringe, after which the tube was shaken vigorously. ¹³C NMR of thereaction mixture was taken, then the tube was brought back into theglovebox where a small amount of [PPN][HCO₃] was added to the solution.¹³C NMR of this mixture was taken, confirming the identity of the upheldcarbonate peak.

Product Distribution Under 1 Atmosphere of ¹³CO₂

[PPN]₂[MoO₄] (30.4 mg, 0.025 mmol) was dissolved in ca. 0.6 mL of CD₃CN,and transferred to a J-Young NMR tube. The tube was brought outside theglovebox, and the solution was submitted to 3 freeze-pump-thaw cycles.After the sample warmed up to room temperature, ¹³CO₂ was introducedwith a syringe. Only the amount of gas pulled in by the vacuum wasintroduced, and the solution was left to equilibrate for 1 minute whilegently shaking the tube, after which the J-Young was sealed. ¹³C NMR ofthis sample under 1 atmosphere of ¹³CO₂ is shown in FIG. 35, at roomtemperature and −19° C. (A: [PPN]₂[MoO₃(κ²-¹³CO₃)], B:[PPN]₂[MoO₂(κ²-¹³CO₃)₂], C: [PPN][HCO₃], and F: free ¹³CO₂). The samplewas then submitted to 3 freeze-pump-thaw cycles in order to remove theexcess ¹³CO₂.

Product Distribution Under 3 Atmospheres of ¹³CO₂

Inside the glovebox, [PPN]₂[MoO₄] (35 mg, 0.028 mmol) was dissolved inca. 0.6 mL of CD₃CN, and transferred to an NMR tube glass blown onto a14/20 female adapter. A vacuum adapter was added, and the sealed systemwas brought outside the box. The solution was degassed using 5freeze-pump-thaw cycles. The system was refilled with ¹³CO2 via syringe,then closed. The NMR tube was placed in liquid nitrogen and flamesealed. The pressure (3 atmospheres) was calculated based on therelative integration of the free ¹³CO₂ and the solvent at roomtemperature, as compared with the corresponding ratio of the sampleprepared under 1 atmosphere as in the previous section. FIG. 36 shows¹³C NMR of [PPN]₂[MoO₄] under 1 atmosphere and 3 atmospheres of ¹³CO₂ atlow temperature (A: [PPN]₂[MoO₃(κ²-¹³CO₃)], B: [PPN]₂[MoO₂(κ²-¹³CO₃)₂],C: [PPN][HCO₃] D: species likely due to binding of ¹³CO₂ to[PPN]₂[Mo₂O₇] at low temperature under excess ¹³CO₂, as evidenced by thespectrum in FIG. 35, and F: free ¹³CO₂).

X-Ray Crystallographic Data

X-ray crystallographic data for the structures shown in FIGS. 1 and 2are summarized in FIG. 38.

Computational Data

Computational Details

Electronic structure calculations were carried out using the M06 densityfunctional with the Def2-QZVPP basis set for molybdenum, incorporatingthe SDD effective core potential, and 6-311+G(3df) for all other atomsas implemented in the Gaussian 09 suite of programs. Minimum energy andtransition state structures were optimized in THF solution using theCPCM model to describe solvation effects. The obtained stationary pointswere characterized by performing energy second derivatives, confirmingthem as minima or transition states by the number of negativeeigenvalues of the hessian matrix of the energy (zero and one negativeeigenvalues respectively). Finally, single-point energies werecalculated with the quadratic configuration interaction method withsingle and double excitation and perturbative corrections for tripleexcitations (QCISD(T)) at the optimized M06 geometries. See, forexample, D. Andrae, U. Haeussermann, M. Dolg, H. Stoll and H. Preuss,Theor Chim Acta, 1990, 77, 123-141; M. J. Frisch et al, Gaussian 09,Revision C.01, Gaussian, Inc.: Wallingford, Conn., 2010; V. Barone andM. Cossi, J. Phys. Chem. A, 1998, 102, 1995-2001; A. Klamt and G.Schuurmann, Perkin Trans. 2, 1993, 799-805; J. A. Pople, M. Head-Gordonand K. Raghavachari, J. Chem. Phys., 1987, 87, 5968-5975; and P. J.Knowles and H. -J. Werner, Chem. Phys. Lett., 1985, 115, 259-267, eachof which is incorporated by reference in its entirety,

XYZ coordinates for all computed species CO₂ C 0.00000000 0.000000000.00000000 O 0.00000000 0.00000000 1.15384300 O 0.00000000 0.00000000−1.15384300 [MoO₄]²⁻ Mo 0.00000000 0.00000000 0.00000000 O 1.018112001.01811200 1.01811200 O −1.01811200 −1.01811200 1.0181.1200 O 1.01811200−1.01811200 −1.01811200 O −1.01811200 1.01811200 −1.01811200 TS1([MoO₄]₂ ⁻ + CO₂ → [MoO₃(κ²-CO₃)]²⁻) C −2.55986400 0.02439000−0.00006200 O −3.27956000 −0.90856700 −0.00011200 O −2.360243001.18712000 0.00020900 O −0.82667800 −0.77271000 −0.00097700 Mo0.82267800 −0.02825700 −0.00003900 O 1.02641700 0.96430300 1.42417400 O1.02747200 0.96632500 −1.42274500 O 2.01343000 −1.30641300 −0.00029700[MoO₃(κ²-CO₃)]²⁻) C −1.97701500 0.02314300 −0.00006400 O −3.199362000.12536000 0.00003500 O −1.30161800 −1.08065000 −0.00025200 O−1.16215000 1.07167300 0.00028700 Mo 0.63401800 0.00248800 0.00007500 O1.17608100 −0.83419200 −1.40906100 O 1.17647200 −0.83319500 1.40963700 O1.46474200 1.52058500 −0.00099300 TS2 ([MoO₃(κ²-CO₃)]²⁻ + CO₂ →[MoO₂(κ²-CO₃)₂]²⁻) Mo 0.34057700 −0.71319200 −0.03143300 O 0.96201000−1.96647100 −1.02020200 O 0.32804900 −1.26071100 1.58205900 O−2.96600900 1.17472800 −1.00799000 O −1.36528300 −0.55856200 −0.54763200O 0.32573100 1.41320100 0.07781900 O 2.16592700 0.31843400 −0.11687200 C1.63073000 1.51676600 0.01087700 C −2.62730000 0.61598200 −0.01939800 O−2.75229700 0.46421700 1.14998900 O 2.26127300 2.55985900 0.05423900MoO₂(κ²-CO₃)₂]²⁻ Mo 0.00001700 0.65798100 −0.00000800 O 0.375062001.68532700 −1.29156000 O −0.37501000 1.68535000 1.29153300 O 1.06193300−1.04391100 −0.94557000 O 1.71440300 0.02671700 0.81582700 O −1.06208800−1.04375800 0.94571600 O −1.71432300 0.02667400 −0.81589600 C−1.98476900 −0.97227800 0.05195100 C 1.98473200 −0.97234400 −0.05192800O 2.98016000 −1.66620200 0.05612300 O −2.98019900 −1.66613200−0.05614600

In order to gain insight into the energetics of this system,computational methods were used (FIG. 39). Binding of the first CO₂molecule is exothermic and exergonic with a ΔH°(298 K)=−14.2 kcal/moland ΔG°(298 K)=−5.2 kcal/mol. The stability of the [MoO₃(κ²-CO₃)]²⁻species is explained by the considerable activation energy of ΔG⁺ ⁺ (298K)=17.2 kcal/mol for regenerating the molybdate with loss of CO₂. Thisis consistent with our inability to remove CO₂ under vacuum at roomtemperature from this material. As expected, binding of the second CO₂is slightly endergonic (ΔG°(298 K)=3.2 kcal/mol), being favored athigher CO₂ pressures and lower temperatures as observed in ¹³C labelingexperiments. The possibility of binding a third CO₂ molecule was alsoinvestigated. However, producing such a species is endergonic with aΔG°(298 K)=14.6 kcal/mol, as well as ΔH°>0 and ΔS°<0. In contrast to thefindings of Mizuno et al. who reported a calculated κ¹ structure for therelated tungstate-CO₂ adduct. (T. Kimura, K. Kamata and N. Mizuno,Angew. Chem. Int. Ed., 2012, 51, 6700-6703 and T. Kimura, H. Sunaba, K.Kamata and N. Mizuno, Inorg. Chem., 2012, 51, 13001-13008, each of whichis incorporated by reference in its entirety) minima corresponding to κ¹structures for any of the molybdenum carbonates studied herein could notbe located.

Reactivity Studies

Catalytic Carboxylation of Epoxides

PPN₂MoO₃CO₃ (155 mg, 0.12 mmol, 1 equiv) was loaded in to a vial,followed by butadiene monoxide (890 mg, 12.7 mmol, 106 equiv). The vialwas placed inside a Parr reactor, then pressurized with 20 bar of CO₂.The reactor was then placed in an oil bath and heated at 100° C.overnight (16 h). The reactor was then cooled to room temperature andthen to 0° C. in an ice water bath. The pressure was vented carefully,the reactor was opened and the reaction mixture was analyzed by ¹H NMR.A 2:1 mixture of starting epoxide to carbonate was obtained, along witha small amount of unidentified byproduct. Turnover number under theseconditions was estimated to be 30.

Carboxylation of Activated Olefins

Reaction of PPN₂MoO₃CO₃ withtrans-1-Methoxy-3-trimethylsiloxy-1,3-butadiene (Danishefsky's diene)

Inside the glovebox, the PPN₂MoO₃CO₃ (51 mg, 1 equiv) was dissolved in0.7 mL of acetonitrile-d₃ and transferred to an NMR tube. The tube wascapped with a septum and brought outside the glovebox, where it wassubjected to 3 freeze pump thaw cycles and backfilled with 1 atm CO₂ bysyringe. The diene (8 μL, 1 equiv) was then added using a microsyringe.¹H NMR was taken after 20 min, showing a mixture of hydrolysis product(4-methoxybut-3-ene-2-one), as well as the two tautomers of the desiredcarboxylated product roughly in a 1:2 ratio (hydrolysis to sum oftautomers). ⁹⁵Mo NMR showed only [PPN][MoO₄SiMe₃] after 5 h.

Reaction of PPN₂MoO₃CO₃ with 4-Methoxyacetophenone triethyl-silylenol

PPN₂MoO₃CO₃ (53 mg, 1 equiv) was dissolved in 0.7 mL of acetonitrile-d₃and transferred to a vial with a stir bar. The 4-Methoxyacetophenonetriethyl-silylenol (26 mg, ˜2 equiv) was dissolved in 0.5 mL ofacetonitrile-d₃ in a vial. Both vials were capped with septa and broughtoutside the glovebox. 20 mL or CO₂ were bubbled through the PPN₂MoO₃CO₃solution, then the silylenol solution was added dropwise at roomtemperature under vigorous stirring. The solution was stirred for 5 min,then part of it was transferred to an NMR tube and analyzed by ¹H and⁹⁵Mo NMR. The ration of starting triethylsilylenol to hydrolysis product(4-methoxyacetophenone) to carboxylated product(3-hydroxy-3-(4-methoxyphenyl)acrylate) by ¹H NMR was 1:2:1. The onlyspecies present by ⁹⁵Mo NMR was the triethylsilylmolybdate.

Formation of Ureas from Aromatic Diamines

1,2-phenylenediamine (11 mg, 1 equiv) and [NEt₄]MoO₃CO₃ (46 mg, 1 equiv)were dssolved in 0.8 mL DMSO-d₆. The solution was transferred to an NMRtube and heated overnight at 110° C. A 1:1 mixture of diamine startingmaterial and 2-benzimidazolone product could be seen by ¹H NMR. The onlymolybdenum species by ⁹⁵Mo NMR was dimolybdate TEA₂Mo₂O₇.

Reaction of [NEt₄]₂MoO₃CO₃ with a Ruthenium Complex2TEA₂MoO₃CO₃+(p-cymene)RUCl₂(PPh₃)→(p-cymene)Ru(CO₃)(PPh₃)+CO₂+TEA₂Mo₂O₇+2TEACl

The molybdate monocarbonate TEA₂MoO₃CO₃ (9 mg, 1 equiv) was dissolved in0.8 mL of acetonitrile-d₃ and this solution was used to dissolve the Rucomplex (p-cymene)RuCl₂(PPh₃) (11 mg, 1 equiv). The resulting red-orangesolution was transferred to an NMR tube and analyzed by ¹H and ³¹P NMRafter 1 hour to confirm conversion of ½ the ruthenium complex to(p-cymene)Ru(CO₃)(PPh₃), and by ⁹⁵Mo NMR to confirm conversion of themolybdate monocarbonate to TEA₂Mo₂O₇.

Synthesis and Structural Characterization of [NEt₄]₂[WO₃(κ²-CO₃)]

[NEt₄]₂[WO₃(κ²-CO₃)] was prepared through a similar method as the Moanalogue. CO₂ was bubbled for 5-10 min through a 0.05M acetonitrilesolution of [NEt₄][WO₄]. After stirring for an additional 20 min, allvolatiles were removed in vacuo to yield the desired product. Singlecrystals were obtained via slow vapor diffusion of diethyl ether into anacetonitrile solution of the tungstate carbonate. See FIGS. 40-41 andTable 1.

TABLE 1 Crystal data and structure refinement for X8_14157.Identification code x8_14157 Empirical formula C₁₇H₄₀N₂O₆W Formulaweight 552.36 Temperature 100(2) K Wavelength 0.71073 Å Crystal systemMonoclinic Space group P2₁/n Unit cell dimensions a = 14.9615(11) Å a =90°. b = 8.3504(6) Å b = 100.0811(17)°. c = 17.1925(13) Å g = 90°.Volume 2114.8(3) Å³ Z 4 Density (calculated) 1.735 Mg/m³ Absorptioncoefficient 5.496 Mm⁻¹ F(000) 1112 Crystal size 0.430 × 0.308 × 0.085mm³ Theta range for data collection 1.985 to 31.503°. Index ranges −21<= h <= 21, −12 <= k <= 12, −25 <= l <= 25 Reflections collected 97602Independent reflections 7036 [R_(int) = 0.0396] Completeness to theta =25.242° 100.0% Absorption correction Semi-empirical from equivalentsRefinement method Full-matrix least-squares on F²Data/restraints/parameters 7036/0/243 Goodness-of-fit on F² 1.041 FinalR indices [I > 2σ(I)] R₁ = 0.0194, wR₂ = 0.0524 R indices (all data) R₁= 0.0216, wR₂ = 0.0538 Extinction coefficient n/a Largest diff. peak andhole 2.318 and −1.435 e Å⁻³

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. An isolated compound comprising a molybdatecomplex of carbon dioxide.
 2. The compound of claim 1, wherein themolybdate complex includes a single molybdenum atom.
 3. The compound ofclaim 1, wherein the molybdate complex includes a single carbonategroup.
 4. The compound of claim 3, wherein the molybdate complexincludes [MoO₃(κ²-CO₃)]²⁻.
 5. The compound of claim 1, wherein themolybdate complex includes two carbonate groups.
 6. The compound ofclaim 5, wherein the molybdate complex includes [MoO₂(κ²-CO₃)₂]²⁻. 7.The compound of claim 1, further comprising a non-coordinating cation.8. The compound of claim 7, wherein the non-coordinating cation isbis(triphenylphosphine)iminium, an ammonium or a phosphonium.
 9. Amethod of making an isolated molybdate complex of carbon dioxidecomprising: exposing a molybdate to carbon dioxide; and isolating themolybdate complex of carbon dioxide.
 10. The method of claim 9, whereinthe molybdate is exposed to greater than one atmosphere of carbondioxide.
 11. The method of claim 9, wherein the molybdate complexincludes [MoO₃(κ²-CO₃)]²⁻.
 12. A method for carbon dioxide fixationcomprising: exposing carbon dioxide to a molybdate in the presence of amild nucleophile to produce a carbon dioxide-transformed product. 13.The method of claim 12, wherein the mild nucleophile is a mild hydridesource.
 14. The method of claim 13, wherein the mild hydride sourceincludes a silane or borane.
 15. The method of claim 12, wherein themild nucleophile is an electron-rich alkene or alkyne.
 16. The method ofclaim 12, wherein the mild nucleophile is a metal hydride or metalalkyl.
 17. The method of claim 12, wherein the mild nucleophile is anamine.